AN AB INITIO QUANTUM‐MECHANICAL INVESTIGATION ON THE ROTATIONAL ISOMERISM IN AMIDES AND ESTERS

The ab initio self‐consistent field molecular orbital method is utilized for the computation of the principal characteristics of the rotational Isomerism in formamide, formic acid and their C‐ and N‐methylated derivatives which may be considered as reasonable models for amide and ester bonds in polypeptide and depsipeptide environments. The calculated barrier to rotation is of the order of 20.3 kcal/mol in formamide and 12.3 kcal/mol in formic acid. The energy difference between the trans and cis form is about 1.8 kcal/mol in favor of the trans form in formamide and about 4.8 kcal/mol in favor of the trans form (see text for notation) in formic acid. In formamide, N‐methylation increases slightly the barrier, while C‐methylation decreases it. In formic acid, O‐methylation decreases the barrier while C‐methylation increases it and also increases the cis‐trans energy difference. The loss of stability of the peptide link upon rotation from the preferred trans‐planar conformation is about 1 kcal/mol for a torsion of 10° and 2 kcal/mol for a torsion of 20°. The theoretical results are in very satisfactory agreement with the most recent experimental data in this field.